In 16 experiments male subjects, age 22.4 +/- 0.5 (SE) yr, inspired CO2 for 15 min (8% end-tidal CO2) or hyperventilated for 30 min (2.5% end-tidal CO2). Osmolality (Osm) and acid-base status of arterialized venous blood were determined at short intervals until 30 min after hypo- and hypercapnia, respectively. During hypocapnia [CO2 partial pressure (PCO2) -2.31 +/- 0.32 kPa (-17.4 Torr), pH + 0.19 units], Osm decreased by 3.9 +/- 0.3 mosmol/kg H2O; during hypercapnia [PCO2 + 2.10 +/- 0.28 kPa (+15.8 Torr), pH -0.12 units], Osm increased by 5.8 +/- 0.7 mosmol/kg H2O. Presentation of the data in Osm-PCO2 or Osm-pH diagrams yields hysteresis loops probably caused by exchange between blood and tissues. The dependence of Osm on PCO2 must result mainly from CO2 buffering and therefore from the formation of bicarbonate. In spite of the different buffer capacities in various body compartments, water exchange allows rapid restoration of osmotic equilibrium throughout the organism. Thus delta Osm/delta pH during a PCO2 jump largely depends on the mean buffer capacity of the whole body. The high estimated buffer value during hypercapnia (38 mmol/kg H2O) compared with hypocapnia (19 mmol/kg H2O) seems to result from very strong muscle buffering during moderate acidosis.